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Grass Structure

Much of this section is derived from, or based on, material from Grasses of New South Wales, 4th edn. (Jacobs, Whalley & Wheeler., 2008)

Vegetative Characters

The vegetative, or non-flowering part of the grass plant is highly variable. Plants can grow as thick tufts or rambling climbers. While vegetative characters are often easily distinguished, in grasses they are often extremely variable, being easily influenced by climatic and nutritional conditions.

Habit

Habit describes the basic 'look' of the plant, which can be fairly variable in grasses. This ranges from basic size characters to others describing the roughness or hairiness of various leafy parts of the plant. Flowering culms are included with the vegetative characters to avoid confusion, so culm height refers to the maximum height of the plant, whether or not it is flowering.

Duration

The life span of grasses is variable. Some may only occur for a short time after rain and their cycles may not be tied to the seasons (ephemeral). Others complete their lifecycle within a year and start from seed every year (annual). Still others persist for two (biennial) or more (perennial) years. This can be a difficult character to determine. If the plant is difficult to pull up and has masses of roots and the remains of old shoots around the base it could well be perennial. If the plant is relatively slender and thin rooted it is likely to be an annual.

Rhizomes and Stolons

Many grasses grow as compact tufts or tussocks, but others produce runners which grow horizontally and root at each node. This is useful to the plant because if a runner breaks it will continue to grow as an individual plant. If the runners occur above the ground they are termed stolons and they are rhizomes if they occur beneath the ground. The nodes of stolons produce green leaves, while small brown scale-like leaves are produced at the nodes of rhizomes.

Origin of Shoots

Shoots are produced from axillary buds which lie in the angle where the stem and leaf-sheath meet. If the new shoot grows up between the stem and leaf-sheath it is termed intravaginal. If it bursts through the base of the leaf-sheath it is termed an extravaginal shoot.

Culms

A culm is the upright stem in the middle of a shoot (or tiller). They are solid at the nodes and often hollow throughout the internode area. In some grasses the internodes of the culm are very short until the culm elongates for flowering. Culms range from unbranched (simple) to many-branched.

Leaves

The grass leaf is an elongated structure arising at the node and consisting of a basal cylindrical sheath that encircles the stem or younger leaves. Above this is a flattened blade with parallel venation.

Leaf sheaths are hollow cylinders split down one side with the margins usually overlapping. Ear-like projections (auricles) are often produced at the top of the leaf sheath. Auricles may be reduced to a hairy edge of the leaf-blade base or may occur on one side of the leaf only. Auricles, although a distinctive feature of many species, are delicate structures and may wither with age. Young leaves should always be examined when dealing with auricles.

A band of mechanical tissue, often pale, occurs at the blade/sheath junction, the abaxial portion of which is termed the collar. (Abaxial refers to the surface facing away from the axis of the plant). An abscission layer may also develop enabling the blade to be shed.

A ligule, in the form of a tongue or flap-like transparent membrane or hairs, is attached on the leaf surface facing the stem (the adaxial surface) at the blade/sheath junction. Ligule type is a particularly useful identification character as it is relatively constant within a genus. Some grasses may lack ligules e.g. Echinochloa crus-galli (Barnyard Grass). Many grasses have translucent, membranous ligules whereas others have ligules fringed with hairs or in the form of a rim of hairs.

At the base of the leaf sheath and the blade are intercalary meristems that, for all but old leaves, allow the leaf to elongate after removal of the blade by mowing or grazing or damage by wind and rain. Intercalary meristems are regions in which cells retain their ability to divide and produce new cells. Because grass leaves mature from the tip of the blade down to the base, the sheath meristem remains active longer than the blade meristem. Intercalary meristems also occur in grasses at the base of stem internodes where activity enables the stem to grow upright after lodging.

Though the shape, texture, folding, hairiness etc. of the leaf-blade may be useful diagnostic features, they are often variable within a species or even on the same plant. In some leaf-blades the midrib is prominent, and often pale-coloured, in others the nerves are equally as conspicuous or inconspicuous. Vernation is often defined as the arrangement of a young leaf in the bud, here we have defined vernation as the arrangement of a leaf when viewed in transverse section (across the leaf). Leaf-blades are often flat but may be rolled or folded, (rolling or folding is a response to water stress in some cases) and some are so narrow as to be bristle-like.

Inflorescence Characters

Basic Inflorescence Characters

The stem of a grass tiller in the reproductive stage terminates in one or more inflorescences or flowering heads. An inflorescence includes all the structures above the last leaf of the culm or base. In some tropical grasses, what appears at first as an inflorescence is a complex collection of branches with a leaf-like structure, a spathe or spatheole, at the base of each individual inflorescence. The branches above the spatheole may be condensed or modified and are often in pairs. The spathes may be coloured or green and may or may not be differentiated into blade and sheath.

Each inflorescence consists of a number of spikelets, which contain the very small flowers, arranged on branches or the main axis. Spikelets, the units of the grass inflorescence, are either attached directly to the branches, in which case they are sessile, or are attached by means of stalks or pedicels. The stem immediately below the inflorescence is the peduncle, which continues as the main axis or rachis after the first branch or spikelet. 'Rachis' is a general term and often used for any axis or branch to which spikelets are attached.

In this key we have separated inflorescences by their basic shape when looked at from a distance. An inflorescence which is a solid pen or popsicle shape when viewed at arms length is termed simple while an inflorescence with obvious branches is a compound inflorescence. Inflorescences with two or more branches all arising from the same point are digitate, or if the branches arise from almost the same point subdigitate.

While an inflorescence may consist of spikelets arranged directly on the rachis, the basic type of grass inflorescence is a compound panicle, usually referred to simply as a panicle, with secondary and often higher order branching. The highest order branch may consist of a spike with spikelets attached directly, while in a raceme the spikelets are attached to the rachis by pedicels. Branches with both sessile and pedicelled spikelets are technically rames but are treated here as racemes. Many inflorescences which appear at a distance to be a single rachis are actually intermediate in type and may be thought of as panicles with reduced branching. If the branches are short and crowded together, the panicle is narrow and described as spike-like, contracted or dense. Reduction in the number of branches can give rise to a primary axis with racemes or spikes.

Spikelet

Each spikelet consists of a number of flowers or florets arranged alternately on either side of a central axis, the rhachilla, above two empty bracts, the glumes. 'Bract' is a general term for a much reduced leaf, particularly small leaves subtending flowers. A grass flower is enclosed by two bracts, the outer one on the side away from the axis termed the lemma and an inner bract named the palea. The bracts, together with the flower, make up the floret.

Spikelets in an inflorescence are usually similar in appearance but two types may occur in the same inflorescence or in different inflorescences. In Lamarckia sterile spikelets, not producing fruits, are mixed with fertile spikelets.

Spikelets of many genera of the tribe Andropogoneae are borne in dissimilar pairs, one member of the pair being fertile (and often sessile) and the other pedicellate, sterile and smaller, these are termed companion sterile spikelets.

Male and female spikelets may occur in separate inflorescences which are often unlike in appearance. In a monoecious plant like Zea mays (Maize), both types of inflorescence are found on the same plant. In Zea mays the male inflorescence is terminal and the female (the cob) is found in the axils of the lower leaves.

In a dioecious plant such as Spinifex sericeus male and female inflorescences are on different plants. In Cleistochloa spikelets with flowers that open normally for pollination (chasmogamous) occur in one type of inflorescence, while cleistogamous spikelets (which do not open) occur in inflorescences enclosed in the leaf- sheaths. Spikelets or inflorescences that occur in two forms are often termed dimorphous.

Many grasses, other than the andropogonoid types mentioned, have spikelets arranged in groups, e.g. Hordeum (Barley).

The degree of maturity of spikelets is likely to vary within a large inflorescence. Often the older spikelets are found towards the top or middle of the inflorescence. It is desirable, and usually possible, to examine spikelets at different stages of maturity when identifying a grass.

Involucre

Spikelets or groups of spikelets may be subtended by various structures that function in seed dispersal. These structures include hairs, bristles, spines, burrs or sterile branches, and are referred to as an involucre.

Rhachilla

This small axis is bent alternately in opposite directions (flexuose or 'zig-zag') and bears the florets, which are in two rows.

Glumes

Each spikelet has two glumes at the base. The glumes never subtend a flower or palea. They may however be modified and vary in size, shape, texture, etc. and one (or rarely both) may be absent. Looking like stripes on the glumes are strands of vascular tissue (veins) termed nerves. The number and prominence of nerves is variable between species, but reasonably constant within species. Glumes may resemble the lemmas above them and it is important, when examining grass inflorescences, to be able to recognise the glumes and thus identify spikelets as units.

Florets

The outermost structure of the floret is the lemma, which usually encloses the other parts. If laterally compressed, a lemma typically has a single ridge or keel running down the middle of the back. Usually visible, and often constant in number for a particular genus or species, are several veins or nerves. Lemmas are frequently green and membranous, especially when immature, and become harder and often less green as they mature. There is considerable variation in size, shape, colour, texture, compression, hairiness and presence of appendages such as awns (see variations). Such variations are particularly useful in identification.

At the base of many florets is a hardened structure, often sharply pointed, that develops at maturity and can be seen clearly when the floret separates from the rhachilla. This callus may be formed from part of the rhachilla as well as from part of the lemma and is useful in seed dispersal, enabling the lemma to adhere to animal skins, and to penetrate the soil once the unit has been dispersed. The sharp callus of Hordeum murinum (Barley Grass) causes blindness in lambs and those of grasses such as Aristida spp. (Wire Grass) considerable damage to animal hides.

Paleas are usually 2-keeled (and 2-nerved) and softer and less variable in structure than lemmas. In the floret, a flat side lies towards the rhachilla while the keels touch the sides of the lemma, thus enclosing the flower. Paleas of bamboos and Oryza sativa (Rice) and their relatives are only partly enclosed in their lemmas.

Flowers

There are no sepals or petals, but two or three small fleshy structures, the lodicules, at the base of the ovary, are considered to represent a reduced perianth. The lodicules are delicate structures lying between the ovary and the lemma (and palea, if a third is present). As the flower opens (anthesis) they swell, pushing the lemma and palea apart to allow first the anthers (usually) and then the stigmas to be exserted (project beyond the bracts) allowing pollination. Protandry is the condition in which the stamens are exserted first and protogyny when stigmas appear before anthers. When stigmas have been exposed for several hours, the lodicules shrivel, allowing lemma and palea to close over the developing grain.

Attached at the base of the flower, in a ring or whorl inside the lodicules, are 1-6 stamens. Most grasses have 3 stamens, with long delicate filaments. The anthers, which open by means of slits, are versatile; that is the filaments are attached at one point above the base. This allows the anther to swing with the wind enabling pollen to be shed in all directions.

The usually elongated superior ovary contains a single loculus or chamber, with one ovule attached near the base on the palea side. At the apex of the ovary are two lateral styles (in some grasses fused to give a single central style) bearing hairy stigmas. In pooid grasses the stigmas are pale while those of panicoid grasses are usually dark (pink to black).

Grain

The ovule of a grass flower develops, after fertilisation, into a seed with much starchy endosperm and an embryo lying towards one side. The ovary wall, which forms the pericarp of the fruit, is normally fused with the seed coat and the fruit termed a caryopsis or grain. Sporobolus and Eleusine produce achenes in which the testa (seed coat) is free from the pericarp. Some bamboos produce berries. Most grass grains are shed enclosed in the lemma and palea. Shape, surface, texture, relative size and position of the embryo are useful diagnostic features.

Variations in Spikelet Structure

Many grass taxonomists consider the basic ('typical') spikelet pattern to be similar to the many-floreted spikelets of grasses such as Avena sativa (Oats). This is thought to have been modified in other species by addition of such structures as awns and hairs etc. and by reduction of the numbers and/or size of spikelet parts.

Additions

Hairs, bristles, spines etc. may be attached to parts of spikelets. In some cases such structures surround individual spikelets or groups of spikelets. The most obvious appendage is the awn.

An awn is a long slender appendage attached to the tip, back or base of a glume, lemma or palea, and the point of attachment is a diagnostic feature. Some awns are spirally twisted, many twisting and untwisting in response to changes in humidity and causing the base of the lemma (often with a sharp callus) to penetrate soil or hides. Such awns are divided into two parts, a relatively stout column and a terminal, slender bristle. A very slender awn is often termed a bristle and a very short, terminal awn a mucro.

Reductions

Both the number and the size of spikelet parts can be reduced. The number of florets in a spikelet varies from one to 20 and, in some grasses, spikelets are reduced to glume(s) only.

If the ovary alone is missing, the floret is termed staminate or male although it is often also called infertile or sterile. A floret without stamens or ovary, with lemma and palea or with lemma only, is termed sterile or neuter, although the terms infertile or empty are also used. A floret with an ovary is described as fertile. The terms sterile, fertile, etc. may be applied as qualifiers to lemmas of appropriate florets.

Although grass florets are normally bisexual, unisexual flowers (both male and female) are not uncommon. They may occur in spikelets with bisexual florets, in single-sex spikelets or in inflorescences with spikelets of one sex only.

The size of glumes, lemmas and paleas may also be reduced. In Microlaena stipoides (Weeping Grass) the glumes are reduced to tiny scales and in Lolium sp. (Ryegrass) one glume is absent on all but the terminal spikelet. The sterile lemmas of some Phalaris spp. may be 1 mm in length.

Other variations

The way in which spikelets break-up at maturity can be a useful diagnostic feature. A joint may be present on the rhachilla above the glumes allowing the florets to fall at maturity leaving the glumes on the pedicel. Alternatively the glumes may fall with the spikelet, in which case the spikelet is said to disarticulate below the glumes. Disarticulation may also take place at joints of the rhachilla and of the rachis. The paired spikelets of grasses belonging to the tribe Andropogoneae usually fall as a unit together with a section of the rachis of the raceme to which they are attached.

The rhachilla is continued beyond the uppermost floret as a small bristle in some pooid spikelets. This 'prolongation' of the rhachilla, often difficult to see, is useful in identification.

The position of reduced florets in a spikelet is also important. Reduced florets may occur on the rhachilla above or below fertile florets.

Several of the variations mentioned tend to occur together and characterise two major groups of grasses. Pooid grasses have one to many florets per spikelet with reduced florets above fertile florets and disarticulation above the glumes. Panicoid grasses exhibit what is often termed a 'reduced' spikelet pattern. In these grasses the floret number is reduced to two, with a sterile (reduced) floret below a fertile floret on a very short rhachilla, and disarticulation is below the glumes. There are of course many exceptions to these generalisations.